In optical disc apparatuses (units) using an optical recording medium, e.g., optical disc, the push-pull method is known as one of the methods for detecting tracking error signal used in the tracking servo control.
FIG. 1 is a view for explaining the principle of the push-pull method. On the base (substrate) of an optical disc 100, grooves 101 and lands 102 are formed in advance as shown in FIG. 1, for example. The push-pull method is a method of detecting, e.g., a light reflected and diffracted at the groove 101 by means of, e.g., a bisected photodetector, 115 of which divided portions are symmetrically disposed with respect to the center of the track to take out an output difference between these detector sections 115L, 115R as a tracking error signal.
In more practical sense, as shown in the FIG. 1 mentioned above, a reflected light from the optical disc 100 mainly consists of (rays of) the 0-th order diffracted light and (rays of) the +.+-.1-th order diffracted light in a direction perpendicular to the groove 101. In view of the above, respective detector sections 115L, 115R of the bisected photodetector 115 are disposed symmetrically with respect to the center of the track to respectively detect, by means of these detector sections 115L, 115R, sum total of intensities of the 0-th order light +(+1)-th order light and sum total of the 0-th order light +(-1)-th order light, i.e., sum total of intensities of the area S.sub.0 where only the 0-th order diffracted light exists and the area S.sub.1 where the 0-th order diffracted light and the 1-th order diffracted light exist and sum total of intensities of the above-mentioned area S.sub.0 and the area S.sub.1 where the 0-th order diffracted light and the (-1)-th order diffracted light exist. Then, a difference between respective outputs of the detector sections 115L, 115R is determined by, e.g., a differential amplifier, thereby making it possible to obtain a tracking error signal.
Meanwhile, in the push-pull method, as shown in FIG. 2, for example, any offset takes place in the tracking error signal resulting from movement of an objective (object lens) 114 followed by tracking (so called fluctuation (change) of the visual field of the objective) and/or inclination in the radial direction of the optical disc, etc.
In more practical sense, as shown in FIG. 3, for example, light beams emitted from a semiconductor laser 111 of the so-called optical pick-up 110 are changed into rays of parallel light by a collimator lens 112, and are then reflected on a beam splitter 113. The (rays of) reflected light thus obtained is converged onto the optical disc 100 by the objective 114. The light beams reflected on the optical disc 100 return through the light traveling (incoming) path in a manner opposite to the above and are transmitted through the beam splitter 113. Thereafter, these light beams are received by the bisected photodetector 115. In this case, in the system of moving only the objective 114 in a direction perpendicular to the optical axis to carry out tracking, when the objective 114 is assumed to be moved to the position indicated by reference numeral 114a, spot of diffracted light would deviate with respect to the center of the bisected photodetector 115. As a result, any d.c. offset might take place in the tracking error signal.
Moreover, as shown in FIG. 4, for example, when the optical disc 100 is assumed to be inclined as indicated by reference numeral 100a, spot of the diffracted light would deviate with respect to the center of the bisected photodetector 115 in a manner similar to the above. Thus, any offset is produced in the tracking error signal. As a result, even if the center of the objective 114 is located on the center of the track, a tracking error signal of which value is not zero is detected, thus failing to carry out precise tracking servo.
Moreover, in optical disc apparatuses adapted for counting the number of tracks that light beams traverse (cross) on the basis of a tracking error signal in the so-called. track jump or seek, if any offset exists in the tracking error signal as described above, it is impossible to precisely count the number of tracks.
In view of the above, in order to eliminate the offset component of the tracking error signal, in the conventional tracking servo apparatus, there is employed a mechanical mechanism such that visual field fluctuation (change) of the objective resulting from, such as, for example, movement of the optical system constituting the optical pick-up or that of the entirety of the optical pick-up, etc. does not take place. In addition, an approach is employed, as shown in FIG. 5, for example, to eliminate the offset component by information except for the tracking error signal, i.e., to wobble the grooves 101 on the basis of address information to cancel (eliminate) the offset component of the tracking error signal, e.g., by difference between amplitudes of wobble (wobbling) components detected by respective detector sections of the bisected photodetector.
However, in both cases, there is the problem that cost is increased for the purpose of realizing them. In addition, in the method of canceling the offset component of the tracking error signal by the wobble component, there is the problem that since it is impossible to precisely detect the wobble component unless the disc apparatus is in the on track state, it is impossible to precisely detect the number of tracks that light beams traverse.
This invention has been made in view of actual circumstances as described above, and its object is to provide a tracking servo apparatus and a track counting apparatus which can lessen offset quantity of the tracking error signal resulting from movement of the objective (object lens) followed by tracking and/or inclination of the optical disc, and which can precisely detect the number of tracks that spots of the light beams traverse.